The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into normally liquid and/or solid hydrocarbons (i.e. measured at 0° C., 1 bar). The feed stock (e.g. natural gas, associated gas, coal-bed methane, residual oil fractions, biomass and/or coal) is converted in a first step into a mixture of hydrogen and carbon monoxide. This mixture is often referred to as synthesis gas or syngas. The synthesis gas is fed into a reactor where it is converted over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
The hydrocarbon products manufactured in the Fischer-Tropsch process are processed into different fractions, for example a liquid hydrocarbon stream comprising mainly C5+ hydrocarbons, and a gaseous hydrocarbon stream which comprises methane, carbon dioxide, unconverted carbon monoxide, unconverted hydrogen, and lower hydrocarbons. The gaseous hydrocarbon stream may also comprise nitrogen and/or argon as the syngas sent to the Fischer-Tropsch reactor may contain some nitrogen and/or argon.
The gaseous hydrocarbon stream is often referred to as Fischer-Tropsch off-gas. Fischer-Tropsch off-gas can be recycled to the syngas manufacturing or to the Fischer-Tropsch reactor. Sometimes lower hydrocarbons are removed before the off-gas is recycled. Lower hydrocarbons may be removed by decreasing the temperature of the off-gas and then applying a gas-liquid separation. However, when the off-gas is recycled to the syngas manufacturing or to the Fischer-Tropsch reactor, the components in the off-gas which do not take part in the reactions, such as nitrogen and argon, occupy reactor space. The components which do not take part in the Fischer-Tropsch reaction are also referred to as “inerts”.
The level of inerts in the Fischer-Tropsch reactor increases with increasing Fischer-Tropsch off-gas recycling. It is common to recycle only a relatively small part of the off-gas. One possibility is to recycle a part of the Fischer-Tropsch off-gas to one or more Fischer-Tropsch reactors and/or to the synthesis gas manufacturing unit, while another part of the off-gas is used as fuel. A downside of this is that only a part of the carbon atoms of the hydrocarbonaceous feed stock is converted to the desired C5+ hydrocarbons. The pace of the build-up of inerts can be reduced by treating the off-gas before it is recycled.
Hydrogen is one of the most valued products. Rather than recovering carbon monoxide, a carbon monoxide shift reactor can be used to increase the hydrogen content of the off-gas.
US20110011128 describes a PSA comprising system in which purified hydrogen is produced using a PSA, which may be a conventional co-purge H2 PSA unit. Such a system may be useful to a hydrogen-rich gas mixture exiting a steam methane reformer, but is not suitable to treat nitrogen comprising hydrogen-lean off-gas of a Fischer-Tropsch process.
US20040077736 mentions a process in which a liquid phase and a vapour phase are withdrawn from a hydrocarbons synthesis stage. In a vapour phase work-up stage, hydrocarbon products having 3 or more carbon atoms may be removed and the residual vapour phase may then pass to a PSA. Using the PSA first, second and optionally third gas components are separated. The first gas component comprises carbon monoxide and hydrogen. The second gas component comprises methane, and the optional third gas component comprises carbon dioxide. The first gas component is recycled to the hydrocarbon synthesis stage. US20040077736 does not provide details on the PSA method used. A regular use of a normal PSA would result in a relatively low recovery of carbon monoxide in the first gas component, and a build-up of nitrogen in the reactor upon recycling the first gas component to the hydrocarbon synthesis stage.
US20080300326-A1 describes the use of a PSA method to separate Fischer-Tropsch off-gas. The method produces at least one gas stream comprising hydrogen, at least one gas stream mainly comprising methane, and at least one gas stream comprising carbon dioxide, nitrogen and/or argon, and hydrocarbons with at least 2 carbon atoms. The PSA used comprises at least three adsorbent beds: alumina, carbon molecular sieves or silicates, activated carbon, and optionally zeolite. The alumina is used to remove water. The carbon molecular sieves or silicates are used to adsorb carbon dioxide and partially methane. The activated carbon is used to adsorb methane and partially nitrogen and carbon monoxide. Zeolite may be used to adsorb nitrogen, argon and carbon monoxide. The product stream of the PSA mainly comprises hydrogen. The other gas streams are obtained during the decompression phase. Disadvantages of the method of US20080300326-A1 are at least the following. Nitrogen is only partially adsorbed in the PSA. This results in a build-up of nitrogen in the Fischer-Tropsch reactor when the hydrogen stream is used, i.e. recycled, as reactant gas. Also the methane stream comprises nitrogen and thus results in the build-up of nitrogen in the syngas, and thus in the Fischer-Tropsch reactor, when the methane stream is used for generating syngas. Another disadvantage of the method of US20080300326-A1 is that carbon monoxide is only recycled to the Fischer-Tropsch reactor in a limited amount. Carbon monoxide is present in the hydrogen stream and in the methane stream.
There is a desire to recover both methane and hydrogen from gas mixtures such as Fischer-Tropsch off-gas in an efficient way. The methane and hydrogen could then, together or separately, be recycled to one or more units in a process line-up such as a Fischer-Tropsch line-up or to a methanol line-up. This would make it possible to convert most of the carbon atoms of the hydrocarbonaceous feed stock to the desired hydrocarbons. It is desired in some cases to additionally obtain a pure carbon dioxide stream from the gas mixture such as a Fischer-Tropsch off-gas, which may be recycled to the synthesis gas manufacturing unit or to a methanol manufacturing facility.